RADAR AND NAVIGATIONAL AIDS
RADAR
Radar is a
device that uses radio waves to track distant objects. The main aim of radar is
to determine: whether there are objects in the region to be searched, the
distance between the radar and the object, and the speed of the object as
needed. Navigation aids are devices that assist vehicles to navigate in areas
where signage is not available, such as at sea or in the air.
Radar-
fundamentals
RADAR stands for RAdio Detection And Ranging. It consists of a transmitter and a receiver, both of which are linked to a directional antenna. The transmitter sends an Ultra High Frequency (UHF) or Microwave signal, while the receiver measures the echo signal returned from the target. When a pulsed signal is utilised in a transmitter, the distance between the transmitter and the target is estimated by calculating the time it takes for the signal to reach the receiver. If a continuous wave is utilised in the transmitter, the target's speed and direction of movement are estimated by detecting the signal frequency difference, which is known as the Doppler effect.
Figure 1.1
shows a pulsed radar block diagram. It is made of a transmitter and a receiver,
both of which are linked by a directional antenna. Through an antenna, the
transmitter may send out UHF or big microwaves.
The receiver
absorbs as much energy as possible from the target's echo, analyses the
incoming signal, and displays it appropriately. The receiving antenna and the
transmitting antenna are the same. Since radio energy is released in pulses,
this is accomplished using a type of time-division multiplexing. The pulse is
sent via the antenna. After some time, the echo signal or signal reflected by
the object reaches the antenna. Because of the time delay, the time allocation
approach is utilized to use the same antenna for transmission and reception.
Applications
·
The
primary applications of radar include, but are not limited to, target search in
open space or at sea, target tracking to follow target trajectory, and aircraft
height readings.
·
Radar
may be utilised as a navigation aid in a variety of ways. It has a wide range
of military applications. Radar technology in aeroplanes can offer important
navigational information. Radar equipment aboard ships gives information on
land masses, other ships, and so on.
·
Radar
is used in the military to deliver armaments to ships, planes, and direct
missiles, among other things. Furthermore, radar is useful for aiding aeroplane
landings, monitoring air traffic at airports, and allowing aircraft to fly
above the ground.
Radar
range equation
The reflected
signal power that reaches the RADAR Receiver diminishes as the distance between
the RADAR Transmitter and the target increases. The RADAR Receiver will be able
to handle a certain amount of detectable power. The minimum power determines
the greatest distance between the RADAR and the target, also known as the
RADAR's Maximum Range. When the received power equals the receiver's lowest
received power Pmin, the maximum range Rmax is reached. Rmax represents the
maximum range. The equation of maximum range Rmax can be given as:
Where Pt = Peak value of the transmitted pulse power.
A0
= capture area of the receiving antenna.
S =
Effective cross-section area of the Target (Also known as Radar cross-section)
Pmin
= minimum received power
λ =
wavelength of the transmitted signal
Factors
influencing Maximum Range
The radar equation is given by
1. As per the
above equation, the maximum range is proportional to the fourth root of the
peak transmitted pulse power. To double the maximum range, the peak power must
be raised 16 times while maintaining all other factors unchanged in the
calculation. Such an increase in electricity is too costly.
2. A decrease
in minimum receivable power has the same impact as increasing transmitting
power and is thus a highly appealing alternative to it.
3. The radar
range equation also demonstrates that the maximum range is proportional to the
square root of the antenna's capture area, and hence exactly related to its
diameter. To double a given maximum radar range, the effective width of the
antenna must be doubled.
4. Increasing
the frequency can also raise the Rmax. There is a limit to increasing
frequency. An antenna's beamwidth is related to the wavelength/antenna diameter
ratio. As a result, every increase in the diameter to wavelength ratio will
result in a narrower beam.
Finally, the
radar equation demonstrates that the maximum radar range is affected by the
target area.
The basic
pulsed radar system
A typical high power pulsed radar system is represented in Fig. 1.2. The modulator is supplied with rectangular voltage pulses by the trigger source. This voltage pulse is utilized as the output tube's supply voltage, switching it on and off.
Microwave
oscillators or amplifiers like klystrons, travel wave tubes, or cross-fields
can be used in these tubes. The radar transmitter section is finished with a
duplexer, which sends the output pulse to the antenna for transmission.
When no
transmission is occurring, the receiver is linked to the antenna. A duplexer is
used for this. In the receiver, the mixer is the initial step. It produces
little noise. The primary receiver advantage is provided at frequencies of 30
or 60 MHz. The IF amplifier is tuned to the same frequency and has the same
bandwidth characteristics as the RF amplifier. Finally, the detector is a Schottky
blocking diode, the output of which is amplified by the same video amplifier as
the IF amplifier. After that, the output is sent to the display unit. A cathode-ray
tube is the most common type of display device.
Display
methods
A radar
receiver's output can be represented in various ways. The three most popular
methods are as follows.
They are:
i) A scope
ii) Plan
Position Indicator (PPI)
iii) Direct
feeding to a computer
Separate
displays may provide additional information such as height, speed, or velocity.
A Scope
display
The display
device works in the same way as a cathode ray oscilloscope. Sweep waveforms are
employed on horizontal deflection plates of cathode ray tubes (CRTs). The beam
steadily travels from left to right across the CRT screen before returning to
its original place.
If no signal is received, the display in scope display A is a horizontal line. The demodulation receiver's output is sent to a vertical deflector plate, which causes the beam in the display to travel vertically, as seen in the figure. 1.3
The target
distance is represented by the displacement from the CRT's left side. The
initial 'blip' is created by a transmitted pulse. The other blip is a
reflection from a nearby item, followed by a sound. Different targets then
appear as big fixtures. The height of each beep correlates to the strength of
the returned echo, while the distance from the reference bar is a measure of
the distance.
Scope
performance is great for tracking since only echoes coming from one direction
are visible.
Plan
Position Indicator (PPI)
• The timing
wave of the sawtooth deflects the point of the cathode ray dramatically
off-centre in this situation, therefore plan position indications are most
often employed for this type of intensity modulation. It is timed with the sent
pulse.
• The
distance out from the centre of the display is proportionate to the target
distance of the radar transmitter's echo production.
• The angular
direction of the sawtooth beam location shows the orientation of the antenna
beam.
The signal
from the receiver output is applied to the control electrode of the cathode ray
tube. The bias voltage at the control electrode is adjusted slightly higher
than the cut-off voltage.
As a result,
a signal with a high amplitude activates the spot. As a consequence, the
target's echo shows as a bright spot with the target's distance and azimuth in
polar coordinates. PPI screens are utilised in search radar and are especially
useful when cone scanning is employed.
Automatic
target detection
Manual radar
performance might be inconsistent or incorrect. For example, the radar
receiver's output is processed in a computer system before it is presented on
the radar screen. Analogue computers can also be utilised to receive and
analyse data, as well as for automated tracking and missile indications. A
computer calculates the object's distance from the radar and speed based on the
reflected signal and displays it on a monitor without the need for human
interaction. These systems are referred to as automatic target detection
systems since they function without human involvement.
NAVIGATIONAL
AIDS
Radar may be
used to help navigation in a variety of ways. It has several military
applications. Radar technology in aeroplanes can give valuable navigational
information. Radar equipment aboard ships gives data on land masses, other
ships, and so on.
Radar is used
in the military to deliver weapons to ships, planes, and direct missiles, among
other things. Furthermore, radar is useful for helping aircraft landings,
monitoring air traffic at airports, and enabling aircraft height above the
ground.
Aircraft
landing systems
One of the
most significant elements influencing the dependability of air travel is the
ability to land an aircraft in poor or no visibility circumstances. Two
electronic systems are typically utilised for aeroplane landing systems. They
are:
(i)
Instrument Landing System(ILS)
(ii) Ground
Controlled Approach(GCA)
Both of these
configurations are blind approach systems. The final landing is typically
performed visually after the electronics system has brought the aircraft out of
the overcast in the proper position to execute a landing.
Instrument
Landing System (ILS)
Figure 1.4
shows the key components of the instrument landing system, which include a
runway finding device, skateboard equipment, and a marking beacon.
Runway localization offers lateral guidance, allowing the aircraft to approach the runway in the proper direction. They are made up of a polarised bidirectionally polarised high-frequency radio network. A set of equations is derived using this radio network, as illustrated in the picture. 1.5. The track location's range differs from that of a long wave radio network.
The radiated
wave in the runway localizer is composed of a single carrier wave. The carrier
wave is concurrently amplitude modulated at 90 and 150 Hertz.
The two patterns in fig.1.5 correspond to the relative intensities of the 90 and 150 Hertz sidebands as a function of direction. The equi-signal course directions are therefore represented by equality in the intensities of the two modulations. In the receiver output, suitable filters separate the two modulated signals, which are then individually rectified and applied with opposite polarity to a zero-centre metre. As a result, metre deflection is absent when tone amplitudes are equal. If the tone intensity of the two signals differs, the stronger signal will deflect the pointer in a direction that indicates the direction in which the aircraft should fly to "correct" its flight.
Fig 1.5 Directional Pattern of
Localizer and Glide Path in ILS.
Marker
beacons are used to indicate position along the localizer route, as seen in
fig.1.4. They are made up of low-power extremely high-frequency transmitters
that excite antenna systems. This antenna arrangement generates fan-shaped
beams. The beams are directed so that the wide dimension of the fan is
perpendicular to the localizer route. Tone modulations and dot-and-dash keying
are used to distinguish the various markers.
The
glide-path equipment offers equi-signal path guidance in the vertical plane,
comparable to the equi-signal path guidance in azimuth given by the localizer.
The ideal gliding angle is between 2 and 5 degrees.
The glide
path signal receiver isolates the two modulation tones, which are then
rectified and applied to a zero-centre metre with opposite polarity.
This
indicator is typically coupled with the localizer indication by housing the two-metre
movements in a common casing in such a way that the localizer and glide-path
pointers are vertical and horizontal when not deflected, respectively. Thus,
any flight adjustments necessary to maintain the set courses in both the
vertical and horizontal planes may be achieved with a fast glance at the
one-meter face.
Ground Controlled Approach (GCA)
Two radars
are used in the ground control approach system. The first is for general
observation and to monitor aircraft traffic patterns around the landing strip.
The second is a high-resolution short-range kit that is intended to practise
landings. This second radar has two displays: one that shows elevation as a
vertical displacement and rotates as a horizontal displacement, and the other
that shows azimuth on the PPI indicator. The first display shows the matching
glide path. The second display indicates the approach direction. The aircraft
to be landed using this method is first brought into position using
surveillance radar before beginning its descent. The controller on the
high-resolution radar set indicator then takes over and occasionally informs
the pilot on what needs to be done to ensure the aircraft is on the intended
glide path. As a result, the aeroplane is "discussed" on a route that
corresponds to the right landing, so that when the clouds breakthrough, it is
in the correct position to visually complete the landing.
If the
aircraft cannot be guided to the proper glide for whatever reason, it is
commanded to abort the landing and return for a second attempt.
Advantage
of GCA
The
ground-controlled approach method has the benefit of requiring no equipment
onboard the aircraft other than a standard radio receiver and allowing the
ground installation to be transportable.
Dis-advantage
of GCA
One of the
drawbacks is that the network contains a lot of human links.